Our work to date shows that dietary zinc deficiency causes rapid atrophy of the thymus and loss of T-cell helper function in the young adult mouse. Delayed type hypersensitivity responses are also impaired in the zinc deficient mouse. These data indicate that zinc deficiency primarily impairs T-cell mediated processes in the young adult mouse. In vitro, mitogenic responses indicated that the residual lymphocytes in deficient mice were functional. Full regeneration and restoration of immunity could be accomplished upon returning the deficient mice to diets containing adequate zinc.